SOP: Determination of Water Vapour Transmission Rate by the Dish Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
The water vapour transmission rate (WVTR) of a packaging material is measured by the dish method: an open dish filled with desiccant is tightly covered with the test specimen and exposed to a controlled temperature and humidity. The desiccant gains weight as water vapour permeates through the specimen, and the steady-state rate of weight gain per unit exposed area gives the WVTR.
Apparatus required
- Open-mouth dishes of non-corrodable light metal (such as aluminium), of size and shape readily accommodated on the pan of an analytical balance
- Template for sealing, to maintain the same test area in each dish
- Analytical balance
- Test chamber maintained at 37.8°C and 90% RH, with air circulation over the assemblies
Reagents
a. Anhydrous CaCl₂ (dried at 200°C) in the form of small lumps — passing through a No. 8 (2.38 mm) sieve and free of fines passing through a No. 30 (595 µm) sieve.
b. Molten wax sealant — a mixture of 60% microcrystalline wax and 40% refined crystalline paraffin wax, or beeswax and resin in equal weights.
Sample preparation
Select the sample so as to represent the material to be tested, of uniform thickness. If the two faces of the material are not uniform (e.g., coated on one side), mark the two sides by distinguishing marks (e.g., I for the coated side and II for the uncoated side); when the two sides are not uniform, prepare 4 dishes — two for each side.
Procedure — Dish method
Fill the dish with the desiccant to within 6 mm of the specimen, leaving enough space so that shaking the dish (done at each weighing) will mix the desiccant
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Cut the specimen and fix it to the dish with the molten wax, using the template so that every dish has the same test area; ensure no leakage of water vapour can occur at or through the edges of the specimen
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Weigh the assembly on the analytical balance and place it in the chamber maintained at 37.8°C and 90% RH, with air circulating over the assembly
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Weigh the assembly at intervals and plot the gain in weight against time; draw a smooth curve through the plotted points
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A straight line that adequately fits the plot of four properly spaced successive points indicates the steady state; the slope of this straight line gives the vapour transmission rate for the test area
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Terminate the experiment when the moisture gain is 10% of the starting weight
Calculation
The printed layout of the formula is garbled; the expression below is built from the book’s own definitions of the symbols:
- Water vapour transmission (WVTR), g/m²/24 h = (g × 24) / (t × a)
Where,
- g = weight gain or loss in grams
- t = time in hours during which the gain or loss was observed
- a = exposed area of specimen in m²
Notes
- The dish method may be applied to glued, heat-sealed or creased samples, but the range over which the permeability can be observed is limited. At 20°C and 85% RH, values between 1 and 250 g per square metre per day can be measured with an accuracy of 10%; hence the method cannot be applied to highly impermeable packaging materials. The range can be extended ten-fold by using heat-sealed flat bags.
- More recent methods of determining the WVTR include the hygrometric method, the coulometric cell method and the IR absorption method.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Annual Book of ASTM Standards, 1972, Part 15, ASTM E 96-66, Procedure E — cited in the book.